HPP Activity 75v1 Faraday s Law Exploration Obtain 50 or 100 turn wire coil bar magnet galvanometer Connect the coil to the galvanometer so that a clockwise current will produce a leftward deflection of the galvanometer needle. You need to be aware of one characteristic of the galvanometer: it reacts slowly. The needle will deflect away from zero when there is a current, but then it can take several seconds to return to zero after the current has decreased to zero. Therefore, while investigating the currents produced by the magnet, if the needle is moving toward zero, most likely the current has dropped to zero. GE 1. Lay the coil flat on the table. Take a bar magnet and move it over the coil. 1. What happens? 2. Is any correlation with what you have to do with the bar magnet and getting a current reading on the galvanometer? Supported in part by NSF-CCLI Program under grants DUE #00-88712 and DUE #00-88780
HPP Activity 75v1 2 Now hold the bar magnet over the coil with the N pole towards the coil, as shown below. S Magnet Coil N 3. Pull the magnet vertically upward at a constant speed. Observe the direction of current and indicate it in the During figure below. Indicate direction and strength of the magnetic field in the interior of the coil. (Double click on figure.) 4. Now hold the bar magnet far from the coil with the N pole pointing towards the coil. Bring the magnet towards the coil at a constant speed. Indicate the direction of current of the strength of the field in the interior of the coil.
HPP Activity 75v1 3 5. Obtain a 50 turn coil and a 100 turn coil with the same radius. Pull the magnet away from each at the same speed. How does the amount of current produced compare? Invention Changing the magnetic field going through a closed circuit induces a current. To state a quantitative law about this situation requires that we define magnetic flux. Suppose a magnetic field of strength B is oriented at an angle φ measured with respect to the normal to a small rectangle of surface area A. The magnetic flux Φ B going through the area is defined as Faraday s law says that changing the magnetic flux going through a closed circuit will produce an induced emf in the circuit. The amount is given by If the circuit has N turns instead of one the induced emf is The circuit with an induced emf acts as if a battery has been placed in the circuit: a current is induced. The direction of the induced current is given by Lenz s Law. The direction of the induced current in a circuit that experiences a magnetically induced emf is such as to oppose the change that is producing it. Application
HPP Activity 75v1 4 Obtain a coil multimeter galvanometer bar magnet GE 2. 1. Measure the resistance of the coil with the multimeter. 2. Lay the coil on the table and connect it to the galvanometer, as in GE 1. Place the bar magnet directly over the coil. Pull the magnet about one meter away while timing how long you take to move it and noting the maximum current. Record the values here. 3. Use Faraday s law to make a rough estimate of the field strength at the end of the bar magnet. GE 3. Conside the following scenarios 1. As the resistance is increased in the left hand loop of the figure below, what is the direction of the induced current in the right hand loop? 2. A current I in the long straight wire of the figure below decreases with time.
HPP Activity 75v1 5 a. Determine the direction of the current induced in the loop next to the wire. b. If the loop were doubled in length, how would the induced EMF change? 3. A copper bar slides to the right at a constant speed along copper rails in the shape of an angle. A uniform magnetic field is directed into the page. a. What is the direction of the induced current in the triangle made by the three conductors? b. Is the EMF induced in the triangle increasing, decreasing or constant?